1. Technical Field
The invention relates to a piston ring made of a carrier material, in particular of steel or a cast material, with a wear-resistant coating consisting of a periodic multilayer system, with each periodicity consisting of at least two individual layers of metal nitrides.                2. Related Art        
The running surfaces of piston rings in internal combustion machines are subject to wear and tear during use. In order to minimize wear and tear, the running surfaces of the piston rings are provided with a protective layer.
From JP 2005-082822 AA and JP 2005-187859 AA a carrier material with a multilayer system has been known which features metal nitride layers. No reference can be found in either prior printed publication with regard to other metallic elements in the individual layers made of metal nitrides.
DE 44 41 136 A1 discloses a pressure ring for a piston of a piston cylinder arrangement of a piston combustion engine. The wear-resistant coating consists of a nitride such as CrN, Cr2N, TiN, Ti/TiN or ZrN. The coating consists only of one single layer. No reference to a multilayer system can be found in DE 44 41 136 A1.
JP 2002-256967 AA discloses a gliding element with a wear-resistant coating. It mentions as an example a piston ring that features a layer on the basis of metal nitrides. In this prior printed publication, no references to any multilayer systems can be found either.
From DE 100 61 749 A1, a wear-resistant coating for piston rings in internal combustion engines has been known that essentially consists of chromium carbides, chromium, nickel and molybdenum. A powder consisting of these components is applied to the piston rings by means of high velocity oxygen fuel spraying. Although such protective layers showed good results with regard to wear and tear, crack resistance—in particular under great stress as it occurs in modern diesel engines—is not satisfactory.
In the area of tools subject to high stress such as, for example, cutting tools, it has long been known to provide multilayer systems as a protective layer. From DE 35 12 986 C2, a wear-resistant coating has been known whose individual layer thicknesses lie within a range of 0.5 to 40 nm. To the extent that a TiN layer is used as an individual layer, it is combined with an additional individual layer made of a carbide or a boride.
This multilayer technology has also found entry into the manufacture of piston rings. From JP 2005-060810 A, piston rings for internal combustion engines have been known that are equipped with a multilayer system whose individual layers have the same metallic component and that differ only in nitrogen contents. The layer thicknesses of the individual layers are indicated as <1 μm. The layers are applied by means of a PVD process, in particular of an electric arc process.
DE 10 2004 028 486 A1 describes a gliding component such as, for example, piston rings for internal combustion engines, that features a coating of several individual layers that consist alternatingly of chromium and chromium nitride. The chromium nitride layers may be made of CrN, Cr2N or mixtures thereof. To avoid any jagged transitions, the coating process is controlled in such a way that the individual chromium nitride layers each have a border of Cr2N and a core of CrN. Each individual layer is at least 0.01 μm thick. The maximum thickness amounts to 10 μm. Total thickness of the coating is indicated as 5 to 100 μm.
However, these known multilayer systems have the disadvantage that crack resistance is not satisfactory.